Musical Source Material Rights

It's helpful if you have an agent and/or an impressive musical theatre writing resume. If I were you, I wouldn't put too much effort into it, since you can't seem to get any response. Also, it's not just permission - you'd need a lawyer or agent to negotiate.

^ That is atypical and not at all the standard. Your source material must not hold much value. If the property you're attempting to secure rights for possesses a higher value, you will be hard pressed to find success in unsolicited appeals and cold calls. You need a "name," you need an attorney and you need money to start. Get a producer or reputable theatre company interested first, then let them attempt to secure the rights (assuming you have no agent/manager or legal representation). Best of luck in this very difficult endeavor that will likely lead to closed doors and disappointment without the proper resources.